The invention relates to a sulfite cooking method for the production of cellulose from materials containing lignocellulose with recovery of the cooking chemicals, especially with series connection of cooking, blowing, pulp washing and bleaching with a multiple thermal treatment of the black liquor for recovery of the cooking chemicals.
The main methods for pulp production, the sulfate process and the sulfite process, suffer from considerable disadvantages. With regard to the amounts of pulp produced, the sulfate process is the dominating one worldwide, since it is applicable to all materials containing lignocellulose, and the pulp produced has good technological properties. An important disadvantage of this process is the air pollution by the offensive odor of reduced sulfur compounds that are generated, on the one hand, plus the difficulties of bleaching, combined with considerable waste-water problems on the other. In contrast to this, sulfite pulps are more easily bleachable on account of the decidedly lower lignin content after cooking, and the better solubility of this residual lignin. The raw material that can be used for sulfite pulping is, however, limited, it requires cleaner debarking of the wood because incomplete disintegration of the bark in cooking results in impurities of the pulp. Furthermore, the technological properties of the pulp are decidedly inferior to those of sulfate pulp.
In the known sulfite cooking processes for pulp production from wood or annual plants the digesting of the lignocellulosic materials is carried out with solutions of sulfites or hydrogen sulfites, having an acid or neutral pH-adjustment. Processes in which the hydrogen sulfite solutions additionally contain sulfur dioxide are referred to as acid bisulfite processes. Of these sulfite cooking processes, the magnesium bisulfite process has acquired considerable importance as an alternative to the calcium bisulfite process, since in this case magnesium oxide and sulfur dioxide, which are washed out from the exhaust gases of the black liquor can be recovered for preparation of fresh cooking liquor. Thus, only small amounts of make-up chemicals have to be added in the recovery to compensate the losses. In comparison to the calcium bisulfite process, the costly acid towers for the production of cooking acid are eliminated, and a substantial part of the total sulfur dioxide emission is avoided by washing the sulfur dioxide out of the exhaust gases from the black liquor burning.
To make the production of pulp less harmful to the environment and to enhance its profitability, it has been proposed to add anthraquinone to the cooking liquor in alkaline cooking processes. As a redox catalyst in alkaline cooking processes, anthraquinone accelerates the delignification and stabilizes the reducing ends of the carbohydrates against alkaline "peeling-off." Pulp in higher yields and of better strength properties are the result. Furthermore, the use of anthraquinone makes it possible also to use the sulfur-free soda process for the production of chemical pulps (Holton, H. H. and F. L. Chopman: Kraft Pulping with Anthraquinone: Laboratory and Full-Scale Mill Trials. TAPPI 60, 11, 49-53 (1977)).
Based on the knowledge of the reaction mechanism of anthraquinone, investigations have been carried through to use anthraquinone in alkaline sulfite processes for pulp production. A substantial drawback of this known process, however, is that the residual lignin content cannot be reduced below Kappa number 40, when at the same time pulp yield and quality must be kept on a certain standard. The production of pulp with a high residual lignin content, however, contradicts the efforts to support the lignin-removing function of the bleaching by intensified delignification in cooking and to reduce environmental pollution by waste bleaching liquor (Ingruber, O. V., M. Stredel and J. A. Histed: Alkaline Sulphite-Anthraquinone Pulping of Eastern Canadian Woods. Pulp Paper Mag. Can. 83, 12, 79-88, 1982; Kettunen, J., N. E. Virkola and I. Yrjala: The Effect of Anthraquinone on Neutral Sulphate and Alkaline Sulphite Cooking of Pine. Paperi ja Puu 61, 685-700, 1979; Rauben- heimer, S. and H. Eggers: Zellstoffkochung mit Sulfit und Anthrachinon, Paper 34, 10, V19-V23, 1980).
To eliminate this disadvantage, a two-stage sulfite cooking method has been proposed involving addition of anthraquinone in the first alkaline stage and addition of sulfur dioxide in the second stage, for further reducing the lignin content of the pulp. Disadvantageous, however, is the long time required for the digestion, amounting about twice that of sulfate cooking, and the technological difficulties arising from the two-stage process (Patt, R. and B. Beck: Integrale Holznutzung bei alkalischen Sulfitverfahren unter Zusatz von Anthrachinon. Mitt. Bundesforschungsan- stalt fuer Forstund Holzwirtschaft, Hammburg, No. 146, 1984, 222-233). Also known is a proposal to digest wood with a mixture of water and alcohol. The disadvantage of this proposal is, however, that especially the dissolution of the lignin of softwoods is only possible to a limited extent. Also, the celluloses produced with a pulps still containing a relatively high residual lignin content have unsatisfactory technological properties and the pressure in the digester results in considerable problems on the large technical scale (Kleinert, T. N.: Organosolv Pulping with Aqueous Alcohol. TAPPI 57, 99-102, 1974).
In further development of the above solution it was therefore proposed to add inorganic chemicals to organic solvents in the form of alcohols. It was suggested to use caustic soda solution in addition to methanol or ethanol and to add anthraquinone to the cooking liquor. The disadvantage of this process, which is still under development, consists in the necessary high cooking temperatures and pressures, the low bleachability of the pulp, and the problems and the great expense involved in the recovery of the chemicals, which has to be performed by burning the black liquor and then causticizing the green liquor (Edel, E.: Das MD-Organosolv- Zellstoffverfahren. Deutsche Papierwirtschaft 1, 39-45, 1984; Nakano, J., H. Daima, S. Hosya and A. Ishizu: Studies on Alkali-Methanol Cooking. Ekman Days Stockholm, 2, 72-77, 1981).
Proven methods for the recovery of the cooking chemicals from a sodium sulfite digestion operate with a pyrolytic decomposition of the evaporated black liquor under certain conditions concerning temperature and oxygen. (Technol., Stockholm (223): 26 pp (1964) and Bjoerkman, A., Proc. Iupac/Eucepa Symp. on Recovery of Pulping Chemicals (Helsinki) 1968 pp 235-265 (Fin. Pulp and Paper Res. Inst. (1969).
The injection of concentrated black liquor in form of a fine spray into a vertical cylindrical pressure reactor while preventing any access of air or oxygen has already been introduced; the wall temperatures of the reactor amounted to between 700.degree. and 800.degree. C. This results in a conversion to finely divided solids in the form of sodium carbonate, some Glauber's salt, and carbon. Sodium sulfide in this case is not contained in the solid residues, and the accompanying organic substances are gassified, while the sulfur in the black liquor occurs in the pyrolysis has as hydrogen sulfide. The hot pyrolysis gases are then separated in a cyclone from the solids, oxidized, and reacted with the fresh liquor. Carbon from the leaching of the solid pyrolysis residue can be burned for additional heat recovery. Such a method of recovering the cooking chemicals is relatively simple. Difficulties consist in selection of a suitable material on account of the high corrosiveness of the reaction products (Barclay, H. G., Prahacs, S., and Gravel, J. J. O., Pulp and Paper Mag. Can. 65 (12): T 553 (1964); Gauvin, W. H. and Gravel, H. J. O, TAPPI 43 (8): 678/1960).
Finally, a method is known for the recovery of cooking chemicals from bisulfite black liquors which uses, instead of an indirectly heated steel reactor, a reactor having a masonry lining, which is heated directly with exhaust gases from an oil burner. The retention time in the pyrolysis reactor amounts to only a few seconds. The sulfur contained in the black liquors is converted to hydrogen sulfide, and the sodium compounds are converted to sodium carbonate. The solid pyrolysis residue contains large amounts of carbon and is first passed through a waste-heat boiler, and the dry powder thus obtained is separated from the gas in a two-stage separator. The pyrolysis gases are cooled in a washer, the water vapor is separated, and the gases afterburned and passed through an additional waste-heat boiler to recover the combustion heat. The solids obtained in the separator are mixed with water, sodium carbonate is leached out, and the sodium carbonate solution is filtered to separate the carbon. The sodium carbonate solution obtained is combined with sulfur dioxide from the afterburning of the pyrolysis gases and used for the preparation of the cooking liquor. Problems with material thickness and selection for the pyrolysis reactor are in this case circumvented. The direct introduction of the hot exhaust gases from the oil burner permits the pyrolysis reactor to be lined with masonry. However, the high content of carbon in the solid pyrolysis residues is a disadvantage. The carbon content in this case can be reduced only by the selection of high reaction temperatures, but this leads to deposits on the reactor walls. Thus also the leaching out of sodium compounds is just as limited as is the recovery of cooking chemicals (Horntvedt, E., TAPPI 53 (11): 2147 (1970)).
The present invention is addressed to the problem of creating a sulfite digesting process for the production of pulp from materials containing lignocellulose, which will combine the advantages of the sulfate and sulfite processes, but will avoid the disadvantages involved in these processes, and especially a process which will make it possible to obtain from cellulose-containing materials of a great variety of origins a high yield of a highly digested pulp of very good strength characteristics, with recovery of the cooking chemicals.